This invention relates to improved water containing refractories (WCR) utilizing the addition of solid organic fibers to reduce or eliminate the tendency of explosive spalling as found in castables, plastics, ramming mixes and mortars.
Explosive spalling of a refractory material sometimes occurs during the initial heating of a water containing refractory wherein the water containing refractory experiences cracking or total disintegration. Explosive spalling is related to steam entrapment in the refractory material. The tendency of the water containing refractory material to explosively spall depends on the type of refractory, the type of binder used in the WCR, the amount of binder, the amount of water, the heating schedule, and the ambient temperature during fabrication. One way to prevent explosive spalling is to use heat-up rates which provide sufficient time for the water and steam to escape the refractory material. Another way to accomplish this is by manually creating channels within the refractory mass before the drying process which provide a means for the water to escape.
If the initial heating rate of the WCR is reduced the tendency of the refractory to explosively spall can be reduced or eliminated. However, heat intensive industries face economic penalties due to extended down-time to initially heat the WCR or face the problem of equipment limitations wherein fast heat-up rates cannot be accommodated. Therefore, it is advantageous to attempt to dry the WCR as fast as possible, i.e., rapid-firing wherein the WCR is dried at a high degrees per hour rate.
Some refractories have been formed with large pre-existing channels either created manually or by the addition of channel forming elements, which make it possible for the moisture to escape at a time preceding and after burnout of the channel-forming elements. Evidently the large preexisting channels were found necessary to remove the water from the WCR. These channels are large in the sense that they are too large (approximately 100 microns or greater) to create capillary action within the WCR and hence rely primarily on diffusion during drying to cause fluid and/or gas flow out of the refractory mass.
It was previously thought that the channel-forming material should be highly absorbent in order to absorb and concentrate the water within the material which forms the channel. There exists spalling resistant refractories incorporating an absorbent channel-forming element, i.e., wheat straw. However, non-absorbent channel-forming material, such as plastic sipping straws, have also been used. When mixed with the refractory mass these materials automatically provided large, internal channels through which the water can escape. In the case of wheat-straw the water can enter its channel by either permeating through the wheat-straw material itself or it can enter one of the two channel openings. In the case of plastic sipping straws, the water must enter through one end of the straw and exit via the other openfaced end. During the drying stage water travels through the channel primarily by diffusive action, due to the pressure differential in the channels since the channels are too large to effect capillary action within the channel. At some temperature over 212.degree. F. (100.degree. C.) the channel-forming material will burn out leaving a larger refractory-defining channel (the channel remaining after the straw has burned out.) Thereafter the entrapped steam will escape the WCR through the refractory-defining channels. WCR made under these conditions experience a substantial loss in strength and a reduced resistance to molten metal or slag attack as compared to conventional WCR.
The use of channels to remove moisture from a refractory mass is discussed in U.S. Pat. No. 3,982,953 (IVARSSON et al) which discloses the addition of straw-shaped, channel-forming elements to a refractory mass for the purpose of automatically creating channels in order to allow moisture to escape. The removal of entrapped water is through these elements by diffusive action. U.S. Pat. No. 2,224,459 (MATHENY) discloses the addition of 1-50% by weight shredded or comminuted paper for the purpose of producing a lightweight refractory. Drying is enhanced by the capillary action created throughout the fiber of the paper. U.S. Pat. No. 3,591,395 (ZONSVELD et al) discloses the addition of polypropylene fibers to a water-hardenable mass for the purpose of resisting material cracking and adding flexural strength to the casting. The mass is not subjected to firing nor are the fibers burned out. The water hardenable mass relies on the presence of the fibers to provide enhanced strength.